Carb'd 5.3 Breaking up at 5k rpm under heavy load
08-24-2015, 09:44 AM
#1
Carb'd 5.3 Breaking up at 5k rpm under heavy load Title pretty much says it all, i built a 5.3 for a jet boat and at WOT which happens to be right about 5k rpm she breaks up. Almost like hitting a rev limiter, but the rev limiter is set at 6400.
It ONLY breaks up after the thing gets good and hot. When its cold it runs clean and pulls very hard all the way to 5-5400. Wont pull more than that due to my setup.
Part throttle response is excellent and as expected, fuel mileage is great. It seems to break up anytime the engine has ran for awhile and part throttle is fine. Just WOT. Seems to happen more if im running the headers dry (hotter) than when i inject the water to cool them off.
Seems to be related to heat.
Here are the specs
5.3 (stock lower end, ARP rod bolts)
243 heads
BTR Stage 4 LS1 cam
Demon 850 with annular boosters
MSD 6012 (8 degrees initial 34 total all brought in around 2k)
Victor JR intake
Long tube over the transom headers, (1 7/8 primaries, 3.5" collectors)
LS1 Coils
Any ideas? Im thinking it may be coils, the plugs are TR6's, wires are brand new GMPP ones.
Since it seems heat related, im thinking it may be related to coils but im open to other ideas. Maybe a crank sensor? Its not new cam sensor is.
It ONLY breaks up after the thing gets good and hot. When its cold it runs clean and pulls very hard all the way to 5-5400. Wont pull more than that due to my setup.
Part throttle response is excellent and as expected, fuel mileage is great. It seems to break up anytime the engine has ran for awhile and part throttle is fine. Just WOT. Seems to happen more if im running the headers dry (hotter) than when i inject the water to cool them off.
Seems to be related to heat.
Here are the specs
5.3 (stock lower end, ARP rod bolts)
243 heads
BTR Stage 4 LS1 cam
Demon 850 with annular boosters
MSD 6012 (8 degrees initial 34 total all brought in around 2k)
Victor JR intake
Long tube over the transom headers, (1 7/8 primaries, 3.5" collectors)
LS1 Coils
Any ideas? Im thinking it may be coils, the plugs are TR6's, wires are brand new GMPP ones.
Since it seems heat related, im thinking it may be related to coils but im open to other ideas. Maybe a crank sensor? Its not new cam sensor is.
08-24-2015, 04:00 PM
#4
Since it is temperature sensiitive, I would say it is likely fuel related. Maybe vapor lock or fuel bowl heat related. Do you have a heat isolator under the carburetor like a wood or phenolic spacer? Are the fuel bowls hot to the touch after a good hard run?
What heat range plugs are in it?
What heat range plugs are in it?
08-24-2015, 04:55 PM
#5
Since it is temperature sensiitive, I would say it is likely fuel related. Maybe vapor lock or fuel bowl heat related. Do you have a heat isolator under the carburetor like a wood or phenolic spacer? Are the fuel bowls hot to the touch after a good hard run?
What heat range plugs are in it?
What heat range plugs are in it?
I verified that after a good run the carb is still cool to the touch. So is the fuel pump/regulator and also the fuel.
On a run with the water turned off to the headers, it starts to act up pretty quickly, about 10 minutes in or so. On a run with the water turned on, it can take a very long time before it starts to do it.
FYI with the water turned on, you can literally touch the headers after you shut the engine down. They are that cool.
It takes a good 15 minutes or so at 4000-5000 rpm under heavy load (nothing really loads these things like a well built pump) before it will start to do it, otherwise just an occasional run to WOT here and there it pulls cleanly all the way and really runs hard.
Im running TR6 plugs, they are relatively new.
Im still leaning towards the coil idea, since they are ontop of the valve covers like OEM and the primaries come up right next to them. I imagine the heat coming off of them is fairly intense. Heres a picture for reference
Also, i have a fuel pressure gauge at the dash, and it doesnt seem to move around at wot.
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08-25-2015, 01:52 PM
#8
Okay after doing a lot of research. i have a few things to try.
going to add a ground that goes from one head to the next. Currently i only have one ground coming from the Battery directly to the Driver side cylinder head. It is 1/0 wire, but ill add one to the other head as well
Going to build a temporary heat shield for the coils, see if this helps
If the heat shield does not work. ill Roadkill it and pull the coils off of my truck and try those. If my Trucks coils work fine, then ill buy a new set of coils
going to replace the crank sensor, since it was already in my block when i bought it. No idea how old it is.
I also ordered a new regulator, just to have a matching set. Now both pump and regulator are Aeromotive
If all the above does not work. I dont know what else it could be
going to add a ground that goes from one head to the next. Currently i only have one ground coming from the Battery directly to the Driver side cylinder head. It is 1/0 wire, but ill add one to the other head as well
Going to build a temporary heat shield for the coils, see if this helps
If the heat shield does not work. ill Roadkill it and pull the coils off of my truck and try those. If my Trucks coils work fine, then ill buy a new set of coils
going to replace the crank sensor, since it was already in my block when i bought it. No idea how old it is.
I also ordered a new regulator, just to have a matching set. Now both pump and regulator are Aeromotive
If all the above does not work. I dont know what else it could be
08-26-2015, 04:21 AM
#9
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turn your carb scoop around.....problem solved
just kidding.
I had a similar problem on my S10, ended up being a bad ground for the 6010. It was just like hitting the rev limiter, not all the time. The stud I had the terminal on had a burr on it, the nut felt tight but wasn't quite loading the ring terminal well enough even though it was snug *doh*
Check box grounds, add your grounds to the heads and inspect all the coils to see if any are cracked, delaminating or have poor connections that moisture could get in.
just kidding.
I had a similar problem on my S10, ended up being a bad ground for the 6010. It was just like hitting the rev limiter, not all the time. The stud I had the terminal on had a burr on it, the nut felt tight but wasn't quite loading the ring terminal well enough even though it was snug *doh*
Check box grounds, add your grounds to the heads and inspect all the coils to see if any are cracked, delaminating or have poor connections that moisture could get in.
08-26-2015, 09:17 AM
#10
turn your carb scoop around.....problem solved
just kidding.
I had a similar problem on my S10, ended up being a bad ground for the 6010. It was just like hitting the rev limiter, not all the time. The stud I had the terminal on had a burr on it, the nut felt tight but wasn't quite loading the ring terminal well enough even though it was snug *doh*
Check box grounds, add your grounds to the heads and inspect all the coils to see if any are cracked, delaminating or have poor connections that moisture could get in.
just kidding.
I had a similar problem on my S10, ended up being a bad ground for the 6010. It was just like hitting the rev limiter, not all the time. The stud I had the terminal on had a burr on it, the nut felt tight but wasn't quite loading the ring terminal well enough even though it was snug *doh*
Check box grounds, add your grounds to the heads and inspect all the coils to see if any are cracked, delaminating or have poor connections that moisture could get in.
Haha! Ive tried it with the scoop forward, backward, and off!
Thats good info, ill try adding the ground from one head to the next.
I do have a question, and this could very well be related.
Currently, my MSD box is grounded directly to the battery. The power side goes through a Fuse box then to the battery directly.
My question is that could it be the power side? Its going through an admittedly cheap fuse panel/box thing with all my other power sources in the back. Should it have its own inline fuse?
thanks
08-27-2015, 04:37 AM
#11
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setup a temporary fused wire directly to the battery and see if it gets better. I have a hard time thinking temperature could affect that though.
another thing, What is your alternator/charging setup. Maybe your alternator is weak and cant keep up at higher rpm with a heavy load and heat.
another thing, What is your alternator/charging setup. Maybe your alternator is weak and cant keep up at higher rpm with a heavy load and heat.
08-27-2015, 11:18 AM
#14
setup a temporary fused wire directly to the battery and see if it gets better. I have a hard time thinking temperature could affect that though.
another thing, What is your alternator/charging setup. Maybe your alternator is weak and cant keep up at higher rpm with a heavy load and heat.
another thing, What is your alternator/charging setup. Maybe your alternator is weak and cant keep up at higher rpm with a heavy load and heat.
the alternator is fine i believe. the only electrical items on the boat are
Fuel pump (fuel pump is relayed)
MSD (and other ignition items)
Bilge pump
Gauges
Starter
Other than that its as stripped down as it gets. No stereo, no accessories ntohing
The alternator is a 12si unit rated at 94 amps i believe. A little overkill for not having any accessories but i think its good. Its brand new, and when running my volt gauge reads 14-15 volts all the time. Never seen it less than that unless the engine is running
08-27-2015, 11:20 AM
#15
Cooling side is simple... very simple... Water comes in from the lake/river through the jet, feeds the engine, then tees off to the headers/over board. Steam lines are ran straight over board
engine coolant temp, is never over 160 (no thermostat, just restricted by a valve if needed)
Engine oil temp ive never seen past 180, normally in the 160 or so range